PROJECT SUMMARY Arboviruses cause serious human disease. Viremia level following arbovirus infection of vertebrates is a critical determinant of viral transmission cycles, global viral spread, and disease severity in individuals. Surprisingly, the factors that dictate viremia following arbovirus infection are poorly defined. We found that multiple arboviruses, including chikungunya (CHIKV), Ross River (RRV), o?nyong nyong (ONNV) and Zika viruses, are cleared from the circulation by phagocytic cells. Experiments in splenectomized mice showed that the spleen is dispensable for arboviral clearance. Instead, virus accumulates in the liver and clearance is independent of natural antibodies and complement factors, suggesting a non-opsonic mechanism. Consistent with this idea, clearance of circulating alphaviruses was blocked by competitive inhibitors of scavenger receptors (SRs) that mediate non- opsonic uptake of non-self and modified-self ligands. Remarkably, we found that single lysine (K) to arginine (R) mutations in the E2 glycoproteins of CHIKV and ONNV (E2 K200R), as well as RRV (E2 K251R), abrogated clearance of circulating alphavirus particles by phagocytic cells, and promoted rapid viral dissemination to distal tissues. Moreover, substitution of CHIKV E2 K200 with a variety of other amino acids also allows for clearance evasion, suggesting a specific interaction between key K residues and a host factor. Ks are targets for post- translational modification (PTM), and mass spectrometry analysis of E2 in virions revealed that CHIKV E2 K200 is ubiquitinated. These experiments have revealed a previously unrecognized pathway that controls arbovirus viremia and dissemination in vertebrates. We hypothesize that PTM of key Ks in viral glycoproteins licenses the capture of circulating arboviruses via SRs expressed on liver Kupffer cells (KCs). In Specific Aim 1, the cell types that capture circulating arboviruses will be defined. We also will determine the role of KCs in viral clearance and dissemination, and the development of anti-viral immunity. Finally, we will evaluate the role of phagocytic cells, and KCs specifically, in the clearance of a genera-spanning panel of arboviruses. In Specific Aim 2, we will define the spectrum of blood-borne arboviruses susceptible to SR-mediated clearance. We will use ELISA and cellular binding assays to determine the murine and human SR(s) that bind virus particles. Using SR knockout mice, we will determine the role of specific SRs in clearance of circulating arboviruses. In Specific Aim 3, we will define the role of E2 ubiquitination in the clearance of circulating CHIKV and RRV. We will use mass spectrometry-based proteomics to determine K residues in arboviral particles that are modified with ubiquitin or other PTMs. Finally, we will use a collection of reverse genetics systems to define the role of specific modified Ks in arboviral clearance from the circulation. This work will provide new mechanistic understanding of arbovirus clearance from the circulation. Elucidating these mechanisms could provide new insight into viral transmission, dissemination, and pathogenesis, identify new risk factors of severe disease, and reveal new therapeutic targets for the treatment of arboviral disease.